For example, in the audio equipment, such as an amplifier, in order to realize a good S/N ratio when a digital signal is generated from an analog signal, a differential AD converter driving circuit is used. In the differential AD converter driving circuit, instead of generating the digital signal directly is from the single analog signal, each (i.e. balanced signal) of a positive-phase signal, which is in phase with the analog signal, and a reverse-phase signal, which is out of phase to the analog signal by 180 degrees, is generated (i.e. balanced signal is generated), and then the digital signal is generated from each of the positive-phase signal and the reverse-phase signal. At this time, the general differential AD converter driving circuit uses a source voltage of 5V. Thus, in generating the balanced signal, it is necessary to superimpose a direct current voltage (or reference voltage) of 2.5V which is about ½ of the source voltage, to each of the positive-phase signal and the reverse-phase signal.
It is possible to use the balanced amplifier, in order to superimpose the direct current voltage to each of the positive-phase signal and the reverse-phase signal, simultaneously with the generation of each of the positive-phase signal and the reverse-phase signal. The balanced amplifier generates the positive-phase signal by inputting the single analog signal to an operational amplifier with a gain of “+1” (i.e. to a non-inverting amplifier or a buffer amplifier), and then generates the reverse-phase signal by inputting the generated positive-phase signal to an operational amplifier with a gain of “−1” (i.e. inverting amplifier). In addition, the balanced amplifier superimposes the direct current voltage to each of the positive-phase signal and the reverse-phase signal by inputting each of the generated positive-phase signal and reverse-phase signal to a reverse-phase input terminal of an operational amplifier on which the direct current voltage is inputted to its positive-phase input terminal. At this time, the balanced amplifier prevents the direct current voltage from returning to the operational amplifiers for generating the positive-phase signal and the reverse-phase signal, by locating a capacitor between the operational amplifier for superimposing the direct current voltage and each of the operational amplifiers for generating the positive-phase signal and the reverse-phase signal.